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What you need to know about calcium

What you need to know about calcium

Harvard Health Letter, April 2003

Calcium is billed as the bone-building nutrient. But some experts argue that we should pay more attention to exercise and vitamin D.

Starting on your 51st birthday, current government guidelines say you’re supposed to consume 1,200 milligrams (mg) of calcium daily. At about that age, both men and women begin to experience osteoporosis, a decline in the density of bones that makes them weaker and more likely to break. In essence, your bone becomes more porous, and calcium supposedly fills in the holes.

But calcium is currently at the center of one of nutrition’s most contentious debates. The critics say there’s little evidence that high intake has more than a marginal effect on bone density and fracture prevention. They say exercise and vitamin D are neglected and more important for bone health. They also argue that dairy foods are a bad overall influence on many people’s diets because of the saturated fat, calories, and other factors. Professor Walter Willett, chair of the nutrition department at the Harvard School of Public Health and a member of the Health Letter’s editorial board, is one of the leading lights in the critical camp.

Naturally, the proponents see the evidence quite differently — and they set the government recommendations, so they’re hardly a fringe group. They say dozens of studies have shown that high calcium intake builds up bone and prevents fractures. And they cite calcium’s other possible benefits, such as modest protection against colon cancer.

So what should you do? For women, 1,200–1,500 mg of calcium daily doesn’t seem to have any drawbacks. For men, though, it may. Studies have shown a possible connection between calcium and prostate cancer. Great Britain set its daily calcium recommendation at 700 mg, which Professor Willett believes is probably closer to the amount that men should be consuming.

Perhaps the most important thing you can do is change how you think about calcium. High intake is not the surefire ticket to bone health that it has been made out to be.

If you’re still worried about getting enough calcium, supplements are inexpensive and free of the saturated fat and added calories that you’d get from dairy products. If you’re going to consume dairy products, pick low-fat and skim varieties.

Food sources

Most of the calcium in the American diet comes from dairy products. Not only do they contain a lot of the mineral, but it’s in a form that’s easy to digest and absorb. An 8-ounce serving of plain yogurt provides about 400 mg of calcium; an 8-ounce glass of milk, 300 mg; and a slice of cheddar cheese, 200 mg.

Vegetables are another food source, although figuring out how much calcium you’re actually getting is tricky. If a vegetable contains oxalic or phytic acid, then the calcium may be poorly absorbed because of the acids. For example, a cup of frozen spinach contains almost as much calcium as a cup of milk, but only a tenth as much is absorbed because of the oxalic acid.

Calcium fortification of everything from orange juice to waffles is making it easier to get a great deal of calcium through diet alone. Breakfast cereals have been fortified for a long time; three-quarters of a cup of the breakfast cereal whole-grain Total contains 1,000 mg. For a list of the calcium content of selected foods, visit our Web site at www.health.harvard.edu/health.

Supplements and vitamin pills

Calcium supplements usually come in 500- to 600-mg tablets. You don’t absorb large doses of calcium as efficiently as you do small ones. Thus, much of a 1,000-mg tablet is going to waste, although unabsorbed calcium in the gut may have benefits. Taking a regular-size pill with calcium-fortified orange juice might also be a waste.

The calcium in most supplements is either in the form of calcium carbonate or calcium citrate. Research shows that they are absorbed equally well with meals, but calcium carbonate is harder to digest than calcium citrate. People are usually advised to take calcium carbonate with or soon after a meal. Calcium citrate can be taken at any time.

Most multivitamin pills contain a relatively small amount of calcium — about 100–200 mg.

Problems with supplements

Acid rebound. Calcium carbonate may cause acid rebound: the stomach overcompensates for the high dose of calcium carbonate, which is alkaline, by churning out more acid. For that reason, people with a history of stomach ulcers are advised that they may not tolerate it and may have to switch to calcium citrate.

Constipation. Calcium supplements can have a mild binding effect but by themselves don’t usually cause serious constipation. But if you’re taking another supplement or medication that binds the stool, the addition of calcium supplements could cause a problem.

Too much calcium. Although it doesn’t happen often, some people have taken so much calcium that it causes hypercalcemia, an above-normal level of calcium in the blood. Hypercalcemia may cause nausea, vomiting, confusion, and other neurological symptoms.

Drug interactions. Large doses of calcium interfere with the absorption of a variety of drugs. You should avoid consuming large amounts of calcium — either in food or as a supplement — within 2–4 hours of taking a tetracycline or quinolone antibiotic. After taking alendronate (Fosamax), risedronate (Actonel), or another one of the bisphosphonate drugs for osteoporosis, you should wait at least 30 minutes before consuming a large amount of calcium. People taking calcium-channel blockers or beta blockers should discuss taking calcium with their doctors because it can reduce the effectiveness of these drugs.

Reading the labels

The Nutrition Facts label on food says % DV, which stands for percent Daily Value. The Daily Value for a nutrient is set by FDA regulations. It’s a single number and not the same as the Recommended Dietary Allowances (RDAs), which come from the Institute of Medicine and often vary with age and sex.

Currently, the Daily Value for calcium is 1,000 mg — the RDA for people ages 31 to 50 — not the 1,200 mg RDA for older adults. So, obviously, if you see a nutrition label that says, for example, a serving contains 20% of the DV, it contains 200 mg of calcium, not 240 mg.

The Supplement Facts label on supplements and vitamin pills is also based on the FDA’s Daily Values, not the RDAs. Although the calcium in those pills comes in several forms (calcium carbonate, calcium citrate, dicalcium phosphate), the amount of milligrams listed on the label is the amount of calcium you’re getting, not the larger compound.

Other ways to keep your bones strong
The DOs...

Exercise. Weight-bearing exercise includes any activity that pits you against gravity: not just lifting weights, but walking, climbing stairs, even dancing. It’s tremendously important to bone health and preventing fractures. Your muscles get stronger and more coordinated, which helps prevent falls. There’s also a direct effect on bone. Working muscle stimulates bone into becoming stronger. If you want to lower your risk of osteoporosis, a short, brisk walk every day might better serve the purpose than a big calcium pill.

Vitamin D. Getting enough vitamin D may be the most important variable in preventing osteoporosis. Vitamin D’s main function in the body is to aid calcium absorption. An analysis of data from the Nurses’ Health Study found that study participants who consume 500 IU of vitamin D daily are 37% less likely to have broken a hip than women who consume 140 IU. (IU stands for International Units, a measure of biological activity.) Neither total calcium nor milk consumption was associated with a lower risk for hip fracture.

The current FDA recommendation (the Daily Value) is 400 IU daily. Studies have shown that up to 50% of older Americans don’t get enough vitamin D. There are several reasons for this. The vitamin’s biologically active form is metabolized when the skin is exposed to the ultraviolet radiation in sunlight. Theoretically, sun exposure can give you all the vitamin D you need. But north of about 40 degrees latitude — the latitude of Philadelphia, Indianapolis, and Denver — the winter sunlight is too weak to produce significant amounts of vitamin D. Even in sunnier climes and times of year, older people tend to spend a lot of time indoors. Moreover, older skin is less effective in making the vitamin even when it’s exposed to sunlight. Sunscreens are another problem: they filter out much of the ultraviolet radiation that produces vitamin D.

Theoretically, you could make up for a shortage of sunshine-generated vitamin D with diet. The problem is that precious few foods contain the vitamin. For practical purposes, it’s limited to several types of saltwater fish. So decades ago, health officials in many northern countries decided to fortify foods with vitamin D. In the United States, milk — but not other dairy foods — was chosen. An 8-ounce glass of milk is supposed to contain 100 IU, although surveys have shown that the actual amount can be a great deal less.

Vitamin K. Your bones also need vitamin K, which is found in green, leafy vegetables.

... and the DON’Ts

Caffeine. If you’ve got a four-or-more cups-a-day coffee habit, cut back for the sake of your bones. High levels of caffeine may stimulate calcium excretion.

Protein. High levels of protein, especially animal protein, alter blood chemistry so calcium leaches out of your bones. But there is no cutoff here, just a general word of caution.

Vitamin A. Several studies have shown a correlation between high vitamin A intake and fracture risk. Some experts say it’s time to stop fortifying milk and breakfast cereal with vitamin A, because the population is getting older and the prevalence of osteoporosis is increasing. Meanwhile, you should avoid multivitamins that contain 10,000 IU of vitamin A, which is twice the recommended daily intake of 5,000 IU — and many experts think that’s too much. Bear in mind that the beta carotene in carrots and other vegetables is not a problem. It is far less biologically active than retinol, the form of vitamin A in many vitamin pills and fortified foods.

Tips for increasing your vitamin D intake

Eat more swordfish, bluefish, salmon, mackerel, or sardines. But swordfish and some species of mackerel are on the list of fish with high mercury levels, so don’t go overboard.

Take a calcium supplement that includes vitamin D. Many brands combine calcium with 200–400 IU of vitamin D.

Take a multivitamin. Multivitamins are a safety net for many nutrients that might otherwise fall through the cracks. Most brands contain 400 IU of vitamin D.

Spend more time outdoors. This is a balancing act, because you don’t want to risk skin cancer in pursuit of more vitamin D production.